The present invention relates to methods, devices, systems, and computer-readable media for transmission (“Tx”) drivers in Serializer/Deserializer (“SerDes”) environments. Tx drivers in SerDes environments must fulfill system and/or user requirements. These requirements ensure proper behavior of the system and the most efficient, or most responsive, Tx driver. One requirement is that transmission via the Tx driver cannot begin until the transmitter is coupled to the end-receiver. The end-receiver often includes a resistor connected to a channel that couples the Tx driver to the receiver. This resistor is known as a “Rx termination.” The Tx driver is only eligible for initialization when the connection between the Tx driver and the receiver is established.
Once the connection is established between the Tx driver and the receiver, the Tx driver is initialized, and the Tx driver may either begin a transmission or revert to a low power state. Frequently, after an initialization the Tx driver is entered into a low power state and held in the low power state for a period of time. When held in the low, or the lowest, power state to preserve power, the nodes of the Tx driver are left floating and are susceptible to drift. The drift of the nodes of the Tx driver results in the nodes holding a voltage that could plausibly be any voltage in a voltage range between approximately zero and a rail voltage of the driver system. For example, the voltage at the nodes while the Tx driver is in the lowest power state could be anywhere from 0 V to VDD.
After the Tx driver has entered a low power state, the Tx driver must resume a high power state prior to a transmission to the receiver. In particular, the output common mode must be re-established to a value that is within a range of the expected settled voltage value. In the SerDes Tx driver industry, re-establishment of the common mode to within a range of this expected settled voltage value should be completed in the shortest time possible. This increases device responsiveness to a “wake up” event. As an example, a wake up event could be, in the realm of smartphones, when a user exits a screen saver by applying a specific gesture to the smartphone's screen. The shorter the time that is required by the Tx driver to re-establish the common mode, the more quickly a device can “wake up.” Some computer expansion bus standards refer to the state of the Tx driver, once it has experienced a wake up event, as an electrical idle state.
Different factors influence the speed at which Tx drivers can re-establish the common mode. One of these factors is the permitted capacitance range for the capacitor coupling the Tx driver to the receiver in the system. This coupling capacitor is referred to as an AC-capacitor. The permitted capacitance range for the AC-capacitor is generally imposed by each computer expansion bus standard. The Tx drivers must be designed so that the driver can function in the worst case scenario: the AC-capacitor value is the maximum of the permitted capacitance range. Although not constant across all computer expansion bus standards, the maximum allowable size for the AC-capacitor may be, for example, approximately 265 nF. Conventional designs of Tx drivers have been developed that are capable of re-establishment of the common mode. However, these conventional designs are only capable of achieving an electrical idle at a slow speed and with a particular circuit configuration not tailored to rapidly achieve electrical idle.
FIG. 1 illustrates a circuit 100 executing a conventional approach to the re-establishment of the common mode. The conventional approach to re-establishment of the common mode sets an H-bridge in the Tx driver to what is referred to herein as a “classical margining mode” (also known as a full margining mode). The H-bridge is modeled by circuit 100 and may include many pull-up segments in parallel with corresponding pull-down segments. For simplicity, circuit 100 is shown with only one pull-up segment and one pull-down segment. The circuit 100 includes several circuit elements: VDD, ground, P-channel MOSFET (metal-oxide-semiconductor field-effect transistor) (“PMOS”) 106, PMOS 108, resistor 128, resistor 130, N-channel MOSFET (“NMOS”) 112, NMOS 114, capacitor 120, chip pin 122, chip pin 124, resistor 118, and ground 126. Circuit elements 106, 108 and 128 are connected to VDD at node 102, correspond to activated pull-up segments of the H-bridge, and have a collective impedance 110. Circuit elements 130, 112 and 114 are connected to ground at node 104, correspond to activated pull-down segments of the H-bridge, and have a collective impedance 116.
The total impedance associated with the H-bridge corresponds to the parallel combination of collective impedance 110 and collective impedance 116. The total impedance associated with the Rx termination is represented by the impedance at resistor 118. Generally, the number of activated pull-up and pull-down segments is selected based on a desired total impedance. Thus, an H-bridge with sixty segments might only need to activate forty segments in order to generate the desired total impedance. In classical margining, the total number of required segments is allocated between pull-up and pull-down segments so that half of the total impedance is contributed by activated pull-up segments, and half contributed by activated pull-down segments. Thus, in classical margining, collective impedance 110 is equal to collective impedance 116, and the common mode voltage of the Tx driver is VDD/2. For example, the collective impedances 110 and 116 may each be configured to approximately Z1=100Ω.
FIG. 2 illustrates a circuit 200, which is a Thevenin equivalent of circuit 100. The circuit 200 includes several circuit elements: ground 234, impedence 232, impedence 218, capacitor 220, chip pin 222, chip pin 224, ground 226, and source voltage 236. When the Tx driver is turned on to output the above-noted common mode voltage of VDD/2, a voltage jump occurs at the Tx driver output due to the high frequency nature of the output impedance change. Because the capacitor 220 is initially at zero volts and cannot charge instantaneously, the capacitor is somewhat transparent to the jump, so that the voltages at both terminals of the capacitor are brought to an initial voltage (V0).
If the impedance of the load (Rx) is matched to the impedance 232 of the Tx driver, then V0 will be VDD/4. This is due to the resistive divider formed by impedances 218 and 232. For example, if the impedance 218 was 50Ω (assuming a parallel combination of 100Ω and 100Ω in FIG. 1) and the impedance of 232 was matched to 50Ω, then the output at pins 222/224 is equivalent to current I multiplied by the impedance 218 (e.g., V0=I*50Ω). The source voltage is equivalent to current I multiplied by the sum of impedances 218 and 232 (e.g., VDD/2=I*(50 Ω+50Ω)). Solving for I in the source voltage equation, I=(VDD/2)*(1/100Ω)=VDD/200. Substituting I into the output voltage equation, V0=I*50Ω=(VDD/2)*(1/100Ω)*50Ω. Accordingly, V0=VDD/4.
After reaching V0, the voltage at the output rises according to an RC curve and eventually settles to a voltage Vs=VDD/2, at which point the common mode has been re-established. This is shown in FIG. 3.
FIG. 3 illustrates voltage at the output of the Tx driver when common mode is re-established using only classical margining. FIG. 3 illustrates a case where the floating nodes of the Tx driver output are at ground or 0 V. It is, however, possible that the floating nodes are instead closer to VDD/2. If that were the case, the time needed to reach the settled voltage Vs would be reduced since the initial voltage would be closer to the settled voltage. Therefore, FIG. 3 depicts the worst case scenario (starting from 0 V). Chart 300 depicts voltage (mV) over time (μs) for charging of the capacitor in accordance with classical margining. In the top curve, the initial voltage jump is to VDD/4 of approximately 300 mV, followed by RC charging to settle at 689 mV. For the top curve, the common mode is reestablished to within 100 mV of the settled voltage in approximately 36 μs. The bottom curve has a lower voltage jump and lower settled voltage since the value of its Vdd is lower.
The currently available methods, systems, and devices that re-establish the common mode for TX drivers are slow. Therefore, there is a need for methods, systems, and devices that are capable of rapidly re-establishing the common mode for Tx drivers. In addition, the functionality to rapidly re-establish the common mode should minimize additional hardware while continuing to meet system requirements.